1. Field of the Invention
This invention relates to the field of ultrasonic imaging and, more specifically, to gas filled liposomes prepared using vacuum drying gas instillation methods, and to gas filled liposomes substantially devoid of liquid in the interior thereof. The invention also relates to methods of and apparatus for preparing such liposomes and to methods for employing such liposomes in ultrasonic imaging applications.
2. Background of the Invention
There are a variety of imaging techniques which have been used to detect and diagnose disease in animals and humans. One of the first techniques used for diagnostic imaging was X-rays. The images obtained through this technique reflect the electron density of the object being imaged. Contrast agents such as barium or iodine have been used over the years to attenuate or block X-rays such that the contrast between various structures is increased. For example, barium is used for gastro-intestinal studies to define the bowel lumen and visualize the mucosal surfaces of the bowel. Iodinated contrast media is used intravascularly to visualize the arteries in an X-ray process called angiography. X-rays, however, are known to be somewhat dangerous, since the radiation employed in X-rays is ionizing, and the various deleterious effects of ionizing radiation are cumulative.
Magnetic resonance imaging (MRI) is another important imaging technique, however, this technique has various drawbacks such as expense and the fact that it cannot be conducted as a portable examination. In addition, MRI is not available at many medical centers.
Radionuclides, employed in nuclear medicine, provide a further imaging technique. In employing this technique, radionuclides such as technetium labelled compounds are injected into the patient, and images are obtained from gamma cameras. Nuclear medicine techniques, however, suffer from poor spatial resolution and expose the animal or patient to the deleterious effects of radiation. Furthermore, there is a problem with the handling and disposal of radionuclides.
Ultrasound, a still further diagnostic imaging technique, is unlike nuclear medicine and X-rays in that it does not expose the patient to the harmful effects of ionizing radiation. Moreover, unlike magnetic resonance imaging, ultrasound is relatively inexpensive and can be conducted as a portable examination. In using the ultrasound technique, sound is transmitted into a patient or animal via a transducer. When the sound waves propagate through the body, they encounter interfaces from tissues and fluids. Depending on the acoustic properties of the tissues and fluids in the body, the ultrasound sound waves are partially or wholly reflected or absorbed. When sound waves are reflected by an interface they are detected by the receiver in the transducer and processed to form an image. The acoustic properties of the tissues and fluids within the body determine the contrast which appears in the resultant image.
Advances have been made in recent years in ultrasound technology. However, despite these various technological improvements, ultrasound is still an imperfect tool in a number of respects, particularly with regard to the imaging and detection of disease in the liver and spleen, kidneys, heart and vasculature, including measuring blood flow. The ability to detect and measure these things depends on the difference in acoustic properties between tissues or fluids and the surrounding tissues or fluids. As a result, contrast agents have been sought which will increase the acoustic difference between tissues or fluids and the surrounding tissues or fluids in order to improve ultrasonic imaging and disease detection.
The principles underlying image formation in ultrasound have directed researchers to the pursuit of gaseous contrast agents. Changes in acoustic properties or acoustic impedance are most pronounced at interfaces of different substances with greatly differing density or acoustic impedance, particularly at the interface between solids, liquids and gases. When ultrasound sound waves encounter such interfaces, the changes in acoustic impedance result in a more intense reflection of sound waves and a more intense signal in the ultrasound image. An additional factor affecting the efficiency or reflection of sound is the elasticity of the reflecting interface. The greater the elasticity of this interface, the more efficient the reflection of sound. Substances such as gas bubbles present highly elastic interfaces. Thus, as a result of the foregoing principles, researchers have focused on the development of ultrasound contrast agents based on gas bubbles or gas containing bodies. However, despite the theoretical reasons why such contrast agents should be effective, overall tile diagnostic results to date have been somewhat disappointing.
New and/or better contrast agents for ultrasound imaging are needed. The present invention is directed to addressing these and/or other important needs.